Techniques described below are known as techniques related to a multilayer wiring board where a plurality of substrates including wiring is laminated.
For example, in a known multilayer wiring board, over at least one face of an inner-layer circuit board on which inner-layer circuits and resin insulator layers that each have a reverse pattern of the pattern of the inner-layer circuit are formed, another inner-layer circuit board is laminated via a prepreg.
Stripline having a configuration in which signal wiring is sandwiched via an insulator between shield wiring to which a fixed potential, such as a ground potential or a power source potential, is applied and another shield wiring is formed as described below for example.
A plurality of substrates, which each include signal wiring on one face of a core material and each include shield wiring on the other face of the core material, are prepared. After that, in a state where the signal wiring of one of the substrates and the shield wiring of another one of the substrates face each other, a prepreg is laid between these substrates. The prepreg is formed by for example, impregnating a woven fabric-like material that includes glass fiber, such as glass cloth, with thermosetting resin that contains adhesive. The prepreg functions as an intermediate layer that bonds the one substrate and the other substrate together. After that, the thermosetting resin in the prepreg is hardened by being heated while pressed so that the substrates are pushed against each other. Accordingly, the plurality of substrates is laminated.
When the signal wiring is constituted of a pair of differential wiring lines that constitute differential wiring, and when for example, a high-frequency signal with a frequency of 10 GHz or more is transmitted to the signal wiring, below-described issues may occur. That is, according to an already-existing manufacturing method of a wiring board, glass fiber in a prepreg may easily move closer to the signal wiring. The glass fiber may thus be positioned near only one of the pair of signal wiring lines, and influence of the glass fiber on electrical characteristics of the one signal wiring line may increase. That is, a difference in relative permittivity around the signal wiring line may be caused between the signal wiring lines. As a result, differences in delay time and characteristics impedance may be caused between the signal wiring lines, and transmission quality may decrease. Such issues become more serious as the frequency of a signal is higher.
The following is a reference document.
[Document 1] Japanese Laid-open Patent Publication No. 8-293676.